AVONEX, a recombinant human interferon-beta product, is an antiviral protein produced by fibroblasts after induction by viral infection. This randomized, double-blinded, placebo-controlled study will determine if AVONEX is beneficial in delaying the onset of clinically definite multiple sclerosis in subjects who have experienced a first and recent onset of a demyelinating event, and who are at high risk for multiple sclerosis. A total of 380 subjects will be treated with IM injections of AVONEX or placebo weekly for up to three years. Six patients were enrolled at this center, and there will be no more recruiting. Followup of these patients will continue.